The present invention relates generally to polymeric microgel particles and to thermosetting coating compositions containing such microgel particles which are useful for coating substrates such as metal surfaces.
In the manufacture of metal containers, such as for example aluminum or steel food and beverage containers, a thin protective coating of a thermoset film is applied to the interior of the container in order to prevent contact of the interior of the container with its contents. Such coatings ideally should exhibit good adhesion to the interior metal surface of the container, particularly when the contents of the container are acidic in nature, good stain resistance, low extractables to prevent contamination of the container contents, and a rapid cure rate for economy of container manufacture. Typical synthetic resins which have been utilized in such coatings include, for example, the heat-curable vinyl, butadiene, epoxy, phenolic, alkyl/aminoplast and oleoresinous based polymers.
In one common method of application, coatings of the aforementioned resins have been applied to the metallic interior of the containers in the form of a solution or dispersion of the heat-curable resin in a volatile organic solvent. Coatings formulated in this manner have in practice proven less than desirable since during the drying and curing cycle of the coating operation, vaporization of the solvent creates significant work place pollution problems which have necessitated the use of costly solvent recovery procedures in order to comply with governmental safety and pollution regulations.
Among the various methods which have been proposed to avoid the use of organic solvents in preparing synthetic resin coatings for metal surfaces is to formulate the coating as an aqueous dispersion.
In commonly assigned U.S. Pat. No. 4,289,811, for example, there is described thermosettable coating compositions useful for coating metal surfaces such as cans which comprise an aqueous dispersion of a heat-curable epoxy resin, an aminoplast resin, and an acrylic type interpolymer comprising the copolymerization product of from about 20 to 90 weight percent of a polymerizable .alpha.,.beta.-ethylenically unsaturated carboxylic acid monomer and from about 10 to 80 weight percent of one or more polymerizable carboxyl-free vinyl monomers. The disclosed aqueous dispersions are prepared by dissolving the aminoplast resin in a water-miscible organic solvent, adding the aforementioned monomers to the solution and then heating the solution to form a polymerization product solution containing the aminoplast resin and the acrylic type interpolymer; admixing the resulting solution with the epoxy resin, neutralizing the admixture with ammonia or an organic amine; and then dispersing the admixture into an aqueous medium to form an aqueous dispersion having a solids content of from about 15 to 40 weight percent.
U.S. Pat. No. 3,908,049 also describes a method for coating metal surfaces using aqueous dispersions of film-forming resins. This method involves preparing an aqueous dispersion containing a mixture of a neutralized water-dispersible carboxylic acid containing polymer having a carboxyl content of at least 8% by weight, a water-dispersible heat-curable thermosetting aminoplast or polyepoxide resin and a water-insoluble, long chain monohydroxy alcohol having 8-36 carbon atoms; applying the aqueous dispersion to a metal surface; and then baking the coating at 350.degree.-450.degree. F. to volatilize the alcohol and cure the coating.
In U.S. Pat. No. 3,960,979, it has further been proposed to coat the interior of food and beverage cans by hot melt spray gun with a fast curing, high solids coating composition comprising a blend of (a) a low molecular weight epoxy resin, (b) a liquid nitrogen resin or phenolic crosslinking agent, (c) a flexibilizing polyol, (d) an inorganic or organic monomeric or polymeric acid which acts both as reactant and catalyst, and (e) optionally a surface modifier such as an acrylic polymer containing acrylic acid.
In addition to the foregoing can coating formulations, various other coating compositions containing acrylic type polymers and/or mixtures thereof with other film-forming resins are known to those skilled in the art.
U.S. Pat. No. 3,492,252 describes latex coating compositions comprising an emulsion polymerized acidic interpolymer having a carboxyl content of from 1 to 30 weight percent, preferably from 3 to 15 weight percent; a monomeric diepoxide or a mixture thereof with a bisphenol-epichlorohydrin condensation product having an epoxy equivalent weight of less than 200; and, optionally, a water-soluble, heat-reactive film-forming material such as an aminoplast or phenol/formaldehyde condensation product. The disclosed compositions are useful as protective films for metal substrates which have been previously coated with pigmented primer, and are prepared by synthesizing via emulsion polymerization techniques a latex of the acidic interpolymer and then adding thereto the epoxy and water-soluble film-forming components, either as is, or in the form of emulsions.
U.S. Pat. No. 3,418,392 discloses thermosetting interpolymer emulsions containing as a crosslinking agent the mixture of a polycycloaliphatic polyepoxide (e.g., 3,4-epoxy-6-methylcyclohexylmethyl 2,4-epoxy-6-methylcyclohexanecarboxylate) and a reactive triazine compound (e.g. hexamethoxymethylmelamine), which are prepared by polymerizing a thermosetting interpolymer containing at least one reactive monomer unit using standard aqueous emulsion polymerization techniques and then adding to the resulting polymer emulsion from about 0.5 to 30% by weight of the crosslinking agent mixture. Such coating compositions are recommended for use in textile print pastes, padding liquor for pigment dyeing of textiles, nonwoven textile impregnation dispersions, and generally as solvent based protective coatings for metal surfaces and the like.
U.S. Pat. No. 3,331,805 describes resin binder compositions for use in treating woven and non-woven fabrics. The disclosed binder compositions comprise a linear copolymer containing at least 50% by weight of a monoethylenically unsaturated organic ester having from 4 to 9 carbon atoms and from about 1 to 5% by weight of an unsaturated carboxylic acid, and a two-component crosslinking system comprising a mixture of equal amounts of an alkylated melamine formaldehyde resin and a diepoxide resin. The binder system may be employed in the form of an aqueous dispersion or solution, or alternatively in the form of a slurry of fibrids obtained by shear precipitation of a dispersion of the resin.
U.S. Pat. No. 4,139,514 discloses translucent to clear, water-borne polymer compositions comprising aqueous, essentially organic solvent-free solutions of interpolymer. Such compositions are prepared by forming a polymer latex by emulsion polymerization of a mixture of (1) acrylic acid and/or methacrylic acid; (2) an acrylate or methacrylate ester; and, optionally; (3) a polymerizable ethylenically unsaturated monomer having a terminal vinyl group. The resulting latexes are then neutralized with a base to solubilize the polymer, yielding the water-borne polymer compositions. Such compositions are recommended for use as coatings or inks.
U.S. Pat. No. 3,118,848 describes coating compositions which are prepared by mixing together a water-soluble salt of a vinyl polymer, and a water-soluble epoxy or polyhydroxy compound. One or more water-soluble phenol-aldehyde or amino resins, notably water-soluble urea-aldehyde or melamine-aldehyde resins, may optionally be included as curing agents where low baking temperatures are contemplated.
U.S. Pat. No. 3,156,740 describes thermosetting acrylic resin compositions adapted for application as coatings to protect metal surfaces from the action of water, soap, grease, light and prolonged heat. Illustrative of the preparation of a thermosetting resin composition, there are co-reacted (a) a copolymer of 2-10% of acrylic acid, 4.5-88% styrene and 9-93% of 2-ethylhexyl acrylate and (b) 1-epoxyethyl-3,4-epoxycyclohexane, and then there is mixed therein (c) 1-epoxyethyl-3,4-epoxycyclohexane, following which there is mixed therein (d) a melamine/formaldehyde resin in an amount of 5-50% by weight based on the total nonvolatile content of the composition.
U.S. Pat. No. 3,215,756 describes heat-curable mixtures of a vinyl polymer with an epoxy compound in the presence of an amino resin. For example, a methacrylic acid/methyl acrylate copolymer is admixed with a polyglycidyl ether of Bisphenol A and a urea-formaldehyde resin in an organic solvent, and then coated on a substrate and baked to a thermoset film.
U.S. Pat. No. 3,403,088 describes water-dispersed coating compositions which can be applied by electrodeposition. The coating compositions contain an at least partially neutralized acrylic interpolymer and an amine-aldehyde condensation product or a polyepoxide or both.
U.S. Pat. No. 3,467,730 describes heat-convertible coating compositions which are prepared from carboxy-containing copolymers, epoxide resins and aminoplast resins. In an example, 37 grams of a 50% copolymer (72% styrene, 20% methyl acrylate and 8% acrylic acid) solution, 6.9 grams of a polygycidyl ether of Bisphenol A and 8.3 grams of a butylated urea-formaldehyde resin were blended, drawn down on glass and cured at 200.degree. C. for 30 minutes.
Various types of microgel particles and coating compositions containing the same have also been developed heretofore by those skilled in the art.
U.S. Patent No. 4,172,066, for example, describes water-swellable, shear resistant microgel particles having a water-swollen diameter of from about 0.5 to about 200 micrometers comprising particles of crosslinked, preferably water-soluble addition polymer. Such microgel particles are prepared by the inverse emulsion or suspension polymerization of water-soluble monomers such as acrylamide or mixtures thereof with other water-soluble monomers such as acrylic or methacrylic acid with a small amount of a crosslinking agent, and may be employed as thickening agents for styrene/butadiene copolymer latexes or as water permeability reducing agents in well drilling applications.
British Patent No. 967,051 discloses crosslinked, polymeric microgel particles, comprising from 95 to 99 mole percent of monoethylenically unsaturated monomer, at least 50% of which is a lower alkyl ester of methacrylic acid, and from 5 to 0.1 mole percent of a crosslinking agent, which are substantially free of uncrosslinked polymer having a viscosity average molecular weight of greater than 200,000 and have a swelling ratio in toluene of from 2 to 6. The disclosed microgel particles are prepared by polymerizing an aqueous emulsion of the monoethylenically unsaturated monomer and crosslinking agent until the reaction is from 80 to 95% complete, and then, following the addition of an agent to inhibit the formation of high molecular weight uncrosslinked material, completing the polymerization reaction. After completion of the polymerization reaction, the resulting microgel particles are filtered and dried, and then formulated into a coating composition by mixing with an organic solvent to form a microsol. Optionally, an organic solvent soluble film-forming resin, such as a polymethacrylate, epoxy or alkyd resin, may also be employed in the microsol coating composition.
U.S. Pat. No. 4,055,607 describes high solids thermosetting acrylic polymer solutions which comprise a thermosetting acrylic interpolymer containing reactive carboxylic acid and hydroxy groups dissolved in an organic solvent; at least 0.5 percent by weight of crosslinked microgel particles; and an aminoplast resin. The crosslinked microgel particles comprise the addition copolymerization product of hydroxy-containing and hydroxy-free monomers and a dispersion stabilizer which is prepared from methacrylic acid, glycidyl methacrylate and the reaction product of poly-12-hydroxystearic acid and glycidyl methacrylate.
Nonaqueous coating compositions containing similar microgel particles are also described in U.S. Pat. Nos. 3,880,796 and 3,652,472.
Despite the various advances and improvements in coating compositions described above, the art has nonetheless continued its search for further improvements in coating compositions suitable for use in coating metal surfaces such as the interior of food and beverage containers. The nonaqueous coating compositions described above, for example, while suitable for some uses, are formulated from organic solvents such as xylene, etc., which are unsuitable for use in food or beverage container applications. Moreover, while the aqueous based coating compositions mitigate somewhat the problems associated with the use of organic solvents, further reductions in the organic solvent content of these compositions and improvements in coating adhesion would be desirable.